I agree completely with Manfred's assessment, but just want to add a
couple of comments. First, these NTC resistors are usually referred to
as "current inrush limiters," and cost about a dollar. Second, while
high current ones usually have a relatively low cold resistance (e,g.
5 ohms), lower current ones (1-2amps) can have a 25-50 ohm cold
resistance. I find the CL-140 type very handy for limiting inrush
current in transmitting tube filaments. I also routinely put one in
the primary winding of boatanchor receiver transformers
Because the devices run hot, I avoid clipping their leads short, and
use Teflon sleeving for insulation. It's best to keep them away from
other components.
73,
Jim W8ZR
Sent from my iPhone
> On Feb 13, 2014, at 6:09 PM, Manfred Mornhinweg <manfred@ludens.cl> wrote:
>
> Brian,
>
>> The little black varistor thingies work quite nicely for eliminating
>> that "THUMMMPP!" in small to medium-size equipment.
>
> I don't think they would, really. They don't have enough resistance to do
> that. Transformer saturation and the consequent magnetic flux through the
> cabinet panels would still happen.
>
>> I don't really know what they are
>
> They are NTC resistors. As they get hot, their resiatance drops. A typical
> one used in a switching supply for a few hundred watts at 230 VAC, might
> have 5 ohm when cold, dropping to less than one ohm when hot.
>
> They are intended basically to avoid blowing the rectifiers with the big
> inrush current of direct line-connected (transformerless) rectifier/filter
> circuits. The resistance in these circuits is so low, that when connected to
> a very low impedance AC supply, the inrush current could exceed the absolute
> maximum transient rating of cheap diodes. Adding such an inrush limiter is
> the cheapest fix for that, also avoids welding any switch contacts, and works
> well except for one little problem: If a brief power cut happens, it might
> last long enough for the capacitors to discharge, but not for the inrush
> limiter to cool down. In that case it won't limit the current to a safe value
> when the service returns, and that creates a risk of blowing the rectifier,
> and explains equipment failures happening during microcuts or due to worn
> power plugs or any loose connections.
>
>> but somebody gave me a whole bunch
>> of them a while back. I have two or three different sizes.
>
> The different sizes have different power dissipation ratings. Within each
> size, several resistance values are available.
>
> They are quite useful, but have their limits too.
>
> Manfred
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|